The invention relates to the microfabrication of gas-filled chambers made from silicon and/or glass wafers using anodic and/or fusion bonding techniques. The method is extremely well-suited for microfabricating optical filter components, such as those used in optical transducers to identify and measure the constituents of anaesthetic and breathing gases in medical applications.
Optical infra-red filters have become key components for many infra-red sensors. For instance, carbon dioxide filters which consist of a hermetically sealed carbon dioxide filled chamber having windows are used in medical respiratory applications. Such filters are typically expensive and their lifetime is normally limited because the sealing of the chamber has to be made at chip level using adhesives, or other suboptimal bonding technique. The invention was developed in an effort to improve the fabrication of optical gas filters and related components such as infra-red radiation sources using wafer level silicon micromachining techniques which have started to become more practical in recent years.
Micromachining techniques have made it possible to fabricate different micromechanical components having structure details with dimensions of the order of micrometers and main dimensions perhaps on the order of millimeters. Micromachining techniques are related to methods used in the manufacturing of semiconductors, for example various structures are formed in silicon crystal directly by etching with the aid of a protecting mask, or by growing different thin films on the surface of the silicon crystal via vaporizing, sputtering, printing or other thin film techniques known to those who manufacture integrated circuits. The assignee of the present application has been involved in the development of micromachining techniques to fabricate various components for infra-red sensors such as the infra-red radiation source assembly disclosed in U.S. Pat. No. 5,668,376 entitled xe2x80x9cDouble Radiation Source Assembly And Transducerxe2x80x9d by Weckstrom, et al., issued on Sep. 16, 1997 to the assignee of the present application, herein incorporated by reference. The ultimate goal of the invention disclosed in the above-referenced patent, as well as the invention disclosed in this application, is to use wafer level silicon micromachining techniques to improve the fabrication process of gas sensors used to measure respiratory gases present in patients"" airways during anesthesia or intensive care. The use of effective micromachining techniques not only leads to miniaturized components, but often also leads to more accurate and durable components.
In one aspect, the invention employs the use of micromachining, etching and bonding techniques to fabricate hermetically sealed gas-filled chambers from silicon and/or glass wafers. The hermetically sealed gas-filled chambers have precise dimensions and are filled with a preselected concentration of gas, thus providing exceptional performance for use as an optical gas filter. The microfabricated gas-filled chambers are also durable. The use of fusion and anodic wafer bonding techniques leads to a completely hermetically sealed chamber which is durable even under conditions in which repeated thermal cycling occurs.
The first step in implementing the invention involves etching one or more cavities (or holes) in one or more glass or silicon wafers. The wafers eventually become part of a chip assembly having one or more hermetically sealed gas-filled chambers after appropriate bonding procedures. Preferably, etching techniques are used to create cavities in a silicon substrate comprised of one or more silicon wafers. Interfaces between aligned silicon wafers are bonded using fusion bonding techniques. The silicon substrate with the etched cavity is then placed within a gas-filled anodic bonding environment. The gas-filled anodic bonding environment contains a selected concentration of gas which is maintained at the anodic bonding temperature Tab and pressure Pab while a glass wafer is aligned on the silicon substrate with the etched cavity. (It may be desirable to replace the glass wafer lid with a silicon wafer having a glass coating.) Voltage is applied to anodically bond the glass wafer to the silicon substrate. During this process, gas from the anodic bonding environment is encapsulated inside of the cavity at the same concentration and pressure that the gas is present in the anodic bonding environment. The composition of the gas within the anodic bonding environment, as well as the concentration and pressure Pab of the gas, is predetermined in order that the composition and concentration of the gas encapsulated within the gas chamber are sufficient for the selected application. For instance, when fabricating a gas-filled chamber which is to be used as an optical gas filter at ambient temperatures (such as used in respiratory gases sensor systems) the anodic bonding environment will typically contain carbon dioxide gas at an inflated pressure such as two atmospheres (i.e. an over-pressurized carbon dioxide environment) thereby allowing the optical gas sensor to obtain a reference signal that exactly matches the absorption spectrum of carbon dioxide. Carbon dioxide gas filters have a strong absorption peak at 4.2 xcexcm. In some embodiments of the invention, the optical path length through the chamber is precisely defined by the thickness of the silicon wafers. In order to obtain maximum absorption, the final anodic bonding is implemented in a bonding environment containing carbon dioxide at a relatively high pressure, thereby allowing more carbon dioxide to be encapsulated in the chamber. Due to the use of carbon dioxide over-pressure in the bonding environment, higher concentrations of carbon dioxide are possible within the chamber, and the chamber can thus be made physically smaller. Depending on the application, the smaller size may significantly simplify fabrication.
Silicon is extremely well-suited for use in carbon dioxide filters because silicon is essentially transparent to radiation having a wavelength xcex=4.23 xcexcm. However, optical transmission losses can occur through the glass and by refraction at the physical interfaces along the optical path. These optical transmission losses can be reduced by thinning the glass wafers through which the radiation passes, and by using anti-reflective coatings such as silicon nitride and silicon dioxide. Silicon dioxide is the preferred coating because silicon dioxide does not affect the quality of fusion or anodic bonds.
As an alternative to anodic bonding a glass wafer lid to the assembly within an over-pressured bonding environment, it may be desirable in some circumstances to enclose the chamber while contemporaneously capturing a preselected concentration of gas in the chamber using a silicon wafer lid. In fabricating such an assembly, it is necessary to chemically treat the silicon wafers. Initial bonding to capture the preselected concentration of gas in the chamber can occur at room temperature. The gas pressure and concentration in the initial bonding environment is substantially the same as the pressure and concentration of the gas captured in the chamber after fabrication. After initial bonding, the silicon wafers are annealed by fusion bonding to strengthen the bond and hermetically seal the chamber. Such a microfabricated structure has the advantage of simplicity, and also has reduced optical transmission losses.
In another aspect of the invention, the above-described techniques are used to microfabricate several gas-filled chambers (e.g. optical gas filters) on a single chip. The fabrication can be accomplished within a single gas-filled bonding environment to create several cells having the same gas composition and concentration. Intentional leaks to the surrounding atmosphere can be micromachined into the chip so that one or more chambers communicate with the atmosphere. In this manner, some chambers on the chip will contain a selected gas (e.g. carbon dioxide gas) at a selected concentration while other chambers will remain open to the surrounding atmosphere. Alternatively, the gas-filled environment bonding technique can be applied in sequence in a variety of gas-filled bonding environments in order that various chambers on the same chip may contain gases of different concentration and/or composition. In the preferred method of implementing this aspect of the invention, it is necessary to saw the wafer used to seal the respective chambers between sequential bonding processes.
In yet another aspect of the invention, the microfabricated gas-filed chamber or plurality of chambers are integrated with one or more infra-red radiation sources on a single chip. The silicon wafer on which the one or more IR sources are microfabricated is preferably used to form part of the chamber which contains the gas. In this manner, an IR source and optical gas filter are efficiently and effectively integrated on a single chip assembly. In many cases, it will be preferred that the single chip assembly include a plurality of laterally disposed IR radiation sources microfabricated on a single wafer and bonded to the remainder of the chip assembly to form the respective gas-filled chambers.